Carbon dioxide laser operating upon a vibrational-rotational transition

ABSTRACT

There is disclosed a laser providing emission of coherent radiation near 10 microns in the far infrared and utilizing transitions between vibrational-rotational levels in carbon dioxide. Also disclosed are beneficial effects from addition of oxygen, water vapor and helium to various forms of such a laser.

United States Patent Chandra K. N. Patel Chatham, NJ.

211 Appi.-No 814,510

[22] Filed Mar. 28, 1969 [45] Patented July 27, 1971 [73] Assignee BellTelephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.CoutinualionJn-part of application Ser. No. 409,682, Nov. 9, 1964, nowabandoned Continuation-impart ol application Ser. No. 474,546, July 26,1965, now abandoned Continuation-impart ol appucation Ser. No. 495,844,Oct. 14, 1965, now abandoned.

[72] inventor [54] CARBON DIOXIDE LASER OPERATING UPON AVIBRATIONAL-RO'IATIONAL TRANSITION 3 Claims, 8 Drawing Figs.

[52] US. Cl 331/945 [5 1] Int. Cl. 1101p 3/22 SEPARA T/ON E Ol/IPAE NT[50] Field ol Search 33 i/94.5

[56] Relerences Cited UNITED STATES PATENTS 3,353,115 11/1967 Maiman33i/94.5 OTHER REFERENCES Legay et al: Comptes Rendus, Vol. 259, pp. 99-102, July 6, 1964. V

Patel et al: Bull. Amer. Phys. Soc. Vol. 9, pg. 500, April 15, 1964.

Primary Examiner-Ronald L. Wibert Assistant Examiner-Edward S BauerAnorneysR. J. Guenther and Arthur J. Torsiglieri ABSTRACT: There isdisclosed a laser providing emission of coherent radiation near 10microns in the far infrared and utilizing transitions betweenvibrational-rotational levels in carbon dioxide. Also disclosed arebeneficial effects from addition of oxygen, water vapor and helium tovarious forms of such a laser.

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WITHOUT 0 FLOURESCENCE A \-\WITH 0 E 8 0 Ci LJ 2 LL 0 Y LASER l960.99cm" 2000- FLOURESCENCE 0 I r 7 e0 000 (GROUND STATE) N2 (GROUNDDISCHARGE v STATE) ELECTRON DENSITY PATENTED JUL27 \sm SHEET 8 BF 8 E350253 62 Ema/$2M HjNNoz mo Q25 mmmiaiu zoCmDmES N mm CARBON DIOXIDE LASEROPERATING UPON A VIBRATIONAL-ROTATIONAL TRANSITION RELATED APPLICATIONSThis application is a continuation-in-part of applications Ser. No.409,682, filed Nov. 9, 1964; Ser. No. 474,546, filed July 26, 1965; andSer. No. 495,844, filed Oct. 14, 1965, all three of which applicationsare now abandoned. C

BACKGROUND OF THE INVENTION This invention relates to gas lasers. A gaslaser is a device which includes a gaseous active medium capable of thestimulated emission of coherent radiation and means for pumping themedium to populate selectively an energy level which is opticallyconnected to a lower energy level, whereby there is established apopulation inversion with respect to such two levels. In a laseroscillator, the overall gain must exceed the losses, including thefractional power output. To achieve oscillations, it is important toenclose the active medium in an optical resonator, from which a portionof the coherent radiation can be extracted. In a laser amplifier anoptical resonator is not necessary; but it is important to makeprovision for introducing a beam to be amplified and for extracting theamplified beam.

The incorporation of the foregoing components into an optical device wasproposed by A. L. Schawlow et a]. in their U.S. Pat, No. 2,929,922,issued Mar. 22, 1960.

The stimulated radiation has several properties which are usuallyreferred to by calling it coherent radiation. The first is a very narrowfrequency spectrum, as well as a desired phase relationship. This aspectof coherency is sometimes termed temporal coherency. The second typicalproperty of the coherent light is its capability for being confined in anarrow beam, which property is sometimes termed spatial coherency. Oneattractive aspect of gas lasers is the high degree of spatial andtemporal coherency that can be obtained.

An attractive aspect of gas lasers consists of the variety of differentprocesses which may be used in the formation and destruction of excitedstates, in order to obtain population inversion between a pair ofoptically connected states. As pointed out in the article by W. R.Bennett, Jr., Gaseous Optical Masers," Applied Optics, Supplement anOptical Masers, Vol. 1, page 24 at 34 (1962), optical maser oscillationhas been produced in gas systems where population inversion wasaccomplished by optical pumping, electron impact, excitation transfer innonelastic atom-atom collisions, and by molecular dissociation inatom-molecule collisions. In addition, in my prior application filedMar. 15, 1963, now Pat. No. 3,411,105, pumping of gaseous atoms byelectron impact is disclosed in discharges in which molecules have beendissociated.

An important deficiency of gas lasers hitherto has been their very lowefficiency, typically small fractions of one percent (1 percent). Oneinteresting approach to this problem is that suggested by J. C. Polanyi,Proposal for an Infrared Laser Dependent upon Vibrational Excitation,Journal of Chemical Physics, Vol. 34, page 347 (1961). Polanyi pointsout that the radiative lifetimes for vibrationally excited states areseveral orders of magnitude greater than for electronic states, thusfacilitating attainment of relatively high concentrations of selectivelyexcited vibrational species. In addition, vibrational excitations cantypically be very efficiently produced. Nevertheless, the specificgases, sodium iodide and hydrogen chloride, suggested by Polanyi are notsuitable choices for providing outputs of coherent radiation.

An object of the present invention is a gas laser of improved efficiencyand power output.

SUMMARY OF THE INVENTION I have discovered that the emission of coherentradiation near microns in the far infrared may be efficiently obtainedfrom transitions between vibrational-rotational levels in carbondioxide. I have also discovered beneficial effects upon efficiency andpower output from addition of oxygen, water vapor and helium. 7

One specifically described species of the invention involves a two-gassystem, one of which serves as the exciting gas to help establish apopulation inversion, and the other as the lasing gas from which thedesired radiation is stimulated; and there is avoided undesiredexcitation of the lasing gas by avoiding any treatment of the lasing gaswhich gives rise to such undesired excitation. Specifically, the lasinggas is not itself subjected to its discharge, in contradistinction withthe usual practice in prior art forms of gas lasers.

Nevertheless, as disclosed in my earlier article Interpretation of CO-Optical Maser Experiments," Physical Review Lettcrs, Vol. 12, page 588(May 25, 1964), an earlier embodiment of my invention, for establishingthe population inversion, employed the electron impact excitation ofcarbon dioxide directly. Such embodiments are attractive for manyapplications because of their simplicity.

Carbon dioxide lasers according to my invention may also be pumped bychemical reactions generating carbon dioxide and vibrationally excitednitrogen.

In this first-mentioned embodiment of the invention in which nitrogenserves as the exciting gas and carbon dioxide as the lasing gas, thenitrogen is excited in a separate chamber and thereafter made to flowtherefrom into an interaction chamber for mixing there with the carbondioxide, which is introduced in an unexcited state. A continuous flow ofboth gases is employed for a continuous operation. The carbon dioxidebecomes excited in the interaction chamber by collision with the excitednitrogen in a highly selective fashion. The interaction chamber ispositioned within an optical resonator which provides the regeneration,to facilitate achieving stimulated emission from the excited carbondioxide.

The excited nitrogen, which is the exciting medium, includes an excitedlevel which has a relatively long lifetime,

typically longer than 0.1 second, and which is not readily destroyed bywall collisions. In addition, as pointed out in the article by Morganand Schiff, The Study of Vibrationally Excited N, with the Aid of anIsothermal Calorimeter," Canadian Journal of Chemistry, Vol. 41, page903 at 909 and 910 (1963), this nitrogen level is closely matched inenergy to a carbon dioxide vibrational-level, which I had found is theupper laser level of the 10.6 micron transition employed in my laser.

With respect to the beneficial effects of other gases, I have discoveredthat the addition of oxygen to the gaseous medium of a carbon dioxidevibrational-rotational transition laser makes feasible the use of highertotal gas pressure in the lasing gas, which in turn results in increasedefficiency and increased power output. Further, I have discovered thatthe addition of water vapor to the laser also increases the efficiencyand power output. By such additions, continuous-wave stimulatedradiation was obtained at 10.6 microns at a power level of about 16watts with an efficiency of more than four percent (4 percent).

1 have discovered still further that a substantial increase in poweroutput and efficiency of a molecular nitrogen-carbon dioxide laseroperating at a wavelength near 10.6 microns can be obtained by addinghelium. Overall efficiencies of about 20 percent and continuous-wavepower outputs of about 75 watts per meter, even for relatively longlasers, are achievable in this way. The overall efficiency and overallpower outputs obtainable are higher than for any other laser, atpresent. In specifically-described embodiments employing helium, thetube containing the gaseous mixture has a diameter in the lasing regionof two inches or more.

I have also found that advantageous materials, transmissive at about 10microns, for Brewster-angle end windows and mirror substrates in suchlasers include crystalline zinc sulfide, barium fluoride or potassiumchloride. Output coupling is advantageously provided through atransmissive central aperture in one reflector.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further describedin the following more detailed description, taken in conjunction withthe accompanying drawings, in which:

FIG. 1 shows schematically a partially cutaway view of an illustrativeembodiment of the invention;

FIG. 2 is a partial energy-level diagram of the N CO system, which willbe useful in describing the invention;

FIG. 3 shows schematically a modification of the embodiment of FIG. 1;

FIG. 4 shows schematically and block diagrammatically a partiallycutaway view of another embodiment of the invention employing oxygen andwater vapor;

FIG. 5 shows an energy-level diagram pertinent to the embodiment of FIG.4;

FIG. 6 shows a modification of the embodiment of FIG. 4;

FIG. 7 shows schematically and block diagrammatically a partiallycutaway view of another embodiment of the invention employing helium;and

FIG. 8 shows another modification of the embodiment of FIG. 1.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS With reference now to thedrawing, the laser shown in FIG. 1 includes a first section where thenitrogen which serves as the exciting gas is treated for the productionof molecules in the vibrational level of the ground electronic state. Inthe embodiment depicted, this involves subjection of the nitrogen to aDC discharge where electron-ion recombinations, atomatom recombinationsand excitation indirectly by electron impact on nitrogen molecules aswell as cascades provide the desired excitation of the nitrogenmolecules. In particular, nitrogen, advantageously of a purity betterthan 99.99 percent, is introduced into the discharge region via an inletport 11 for continuous flow, for example, at a linearflow rate of theorder of 500 centimeters per second. The discharge is confined withinthe multisection glass tube 12. In the design being describedspecifically, this tube has an innerdiameter of about millimeters. Thedischarge region in this tube is defined by the cathode l4 and the pairof anodes 15. The use of two spaced anodes permits elongation of thedischarge path without a corresponding increase in the DC voltagerequired. A DC voltage source (not shown) supplies the power necessary.For the design being described involving a separation of about 60centimeters between the cathode and each anode, 2,000 volts with acurrent flow of 9 milliamperes proved satisfactory. Such voltages andcurrents provide a discharge that is substantially cooler than thedischarge employed in my above-cited U.S. Pat., No. 3,41 1,105, andproduces much less molecular dissociation in the discharge.

The now excited nitrogen is made to flow out of the discharge regioninto the interaction region, into which is also introduced unexcitedcarbon dioxide, also advantageously of high purity, by way of inlet port17 and distribution tube 18. The interaction region is enclosed within aglass tube 20. In the design being described, this tube had an innerdiameter of about millimeters. To improve the mixing of the excitingnitrogen and the lasing carbon dioxide, the nitrogen and carbon dioxideare each introduced into the interaction region at a number of separateinlets 21, spaced apart along the interaction region as shown. To keepsmall the amount of atomic nitrogen introduced into the interactionregion, and thereby to improve the efficiency of the interacting, thedistance between the discharge region and the point at which nitrogenenters the interaction region advantageously is made in each instance atleast 6 centimeters to provide time for substantial completion of allthe atomic and electron-ion recombination in the nitrogen before mixingwith the carbon dioxide. In the design discussed, the carbon dioxide andnitrogen flow rates were adjusted to be nearly equal with a total gaspressure of 0.8 Torr in the interaction region.

The spent gases are withdrawn from the interaction region by way of theexhaust port under the action of a pump 31 which controls the flow.Advantageously, as shown, the mixture evacuated is supplied to apparatusshown schematically for separating the two components and afterseparation returning the separate components to the appropriate inletports of the system. The separation apparatus has not been shown indetail as the techniques for such separation are known. Typically,separation would involve passing the mixture through a cold trap forsolidification of the carbon dioxide.

To achieve oscillation, it is important to enclose the interactionregion in an optical resonator. In the specific design being discussed,the resonator was formed by a pair of near confocal concave mirrors 33,34 spaced apart about 1.4 meters and supported by suitable mirror mounts35, 36, respectively. The mirrors are illustratively pressedpolycrystalline zinc sulfide coated to be opaque with vacuum-depositedgold, and energy was coupled out from the resonator through a 0.5millimeter aperture in the center of mirror 33.

If the laser be intended for use as a straight through amplifier theresonator becomes unnecessary and the mirrors can be eliminated. In someinstances, it may be preferred to locate the mirrors outside theinteraction region, in which case Brewster-angle windows of materialtransmissive at. 10.6 microns, for example, pressed polycrystalline zincsulfide, barium fluoride or potassium chloride, can be used to confinethe interaction region.

A discharge in low pressure nitrogen results in very efiicientproduction of nitrogen molecules in the vibrational levels of theirground electronic state. Since nitrogen has a zero permanent dipolemoment, such molecules cannot decay to the v=0 vibrational level throughelectric dipole radiation. Thus, the effective lifetimes of these statesare governed primarily by deactivation through collisions with othermolecules and walls, and accordingly can be high, for example, severalseconds.

In FIG. 2, there is shown the vibrational level v=1 for nitrogen withrespect to the ground state of nitrogen. The other vibrational levelshave been omitted-for simplicity as unimportant. There have also beenshown the vibrational energy levels of carbon dioxide of interest, withrespect to its ground state (000). It can be seen that N, (v=l) at2330.7 cm. is in very close coincidence with the (00l) vibrational levelof CO, at 2349.16 cm.. Thus, a collision of the second kind can have alarge cross section because of the near-perfect coincidence. The lowerlaser level (100) on the other hand is removed from N, (v=l by more than900 cm. and hence the cross section for exciting the CO ground statemolecules to the lower laser level will be very much smaller. Moreover,the excitation of CO, (000) molecules to level (100) involves a reactionwhich includes transitions which are optically forbidden. Thus, when theexcited nitrogen molecules are allowed to mix with the carbon dioxidemolecules, a selective excitation of C0, ground state molecules to the(00l) level takes place. Moreover, since the lifetimes of the (00l)levels are longer than the lifetimes of the (100) levels, the requiredconditions for laser action on the (001)--(100) transitions aresatisfied. The particles thereafter relax from the (100) level to theground state in cascade fashion as depicted schematically. The decayprocesses which are most important are the ones which involve collisionswith other gas particles.

Laser oscillations were obtained at a number of wavelengths involvingP-branch rotational transitions from P( 14) to P(26). The strongesttransition, corresponding to 1*(20), occurs at 10.5915 microns, andcontinuous wave power in excess of l milliwatt was readily obtained withless than 20 watts input power. As disclosed in my above-cited articleof May 25, 1964, laser oscillation was also obtained onvibrational-rotational transitions of the carbon dioxide at 9.6 microns.

There are a large number of reactions through which one can producevarious diatomic molecules in vibrationally excited levels and hence theselective excitation through vibrational energy transfer is applicablefor obtaining laser action on vibrational-rotational transitions ofpolyatomic molecules in other mixed systems. Molecules in vibrationallyexcited levels are of special importance for both the exciting and thelasing gases because of the longer lifetimes available with suchmolecules. Another requirement on the exciting gas is that its excitedmolecules not be easily destroyed by wall collisions, making it feasiblefor transport of these molecules from the discharge region to theinteraction region without excessive destruction. In some instances, itmay be advantageous to treat the walls to minimize their effect on thelifetimes of incident excited particles. Also, it is advantageous thatthe molecule exhibit no permanent dipole moment so as to avoid radiativetransitions between vibrational levels. This ordinarily means that themolecule of the exciting gas should be composed of like atoms.Additionally, of course, there is required the close coincidence betweenthe vibrational level of the exciting gas and an upper level of thelasing gas, which level is optically connected to a lower level.

The specified conditions are also met, for example, in an N -N O systemand an 0,-CS system (the first gas being the exciting gas and the secondthe lasing gas). In particular, the apparatus described has been used togenerate oscillations in the N N O system at 10.8422 microns and otherwavelengths.

It is a further advantage of lasers of the kind described that due tothe absence of any discharge in the interaction region, the usualinverse relation of optical gain with diameter of the interaction regionis typically not applicable here. Thus, the diameter of the interactionchamber can be increased with little adverse effect on the optical gain.Rather, because an increase in diameter can be translated into anincrease in number of excited molecules, the optical power output can beincreased by increasing the diameter of the interaction chamber.

Various other techniques are feasible for the excitation of the excitingmolecules. Such modifications include the use of a radio frequencydischarge rather than a DC discharge and, most simply, heating theexciting gas to populate the desired v=1 level. For example, withnitrogen at l440 K., the population corresponding to the Boltzmanndistribution in the v =1 vibrational level is about percent (10 percent)of the total nitrogen population, which is more than adequate forachieving lasing action with carbon dioxide. Such heating can beachieved simply by flowing the nitrogen past a heating element beforeintroduction into the interaction region. In FIG. 3 such heating isillustratively provided by a heating element 44 of known type disposedwithin the region defined by tube 12. Heating element 44 may, forexample, be energized from an electrical current source 45.Additionally, chemical reactions, for example, as described in the paperentitled Vibrational Disequilibrium in Reactions Between Atoms andMolecules," Canadian Journal of Chemistry, Vol. 38, page 1769 et seq.1960), can be employed to populate vibrational levels.

A modification of the embodiment of FIG. 1 excited by a chemicalreaction of the type disclosed in the just-cited article is shown inFIG. 8. A hydrocarbon is burned in air in combustion chamber 53, whichfor example could include a common Bunsen burner, and caused to flowthrough ducts l2 and 17 by a pump or, advantageously, an expansionnozzle 54. It is known that hydrocarbon-air flames reach a temperatureof about 2500-3000 C. The flame products include CO and N both of whichare vibrationally excited because of the high temperature of the flame.Expansion nozzles are well-known means for causing heated gases to flowand reduce the molecular and rotational temperature of molecules at thesame time but leave the vibrational temperature unafiected which is adesired condition for laser action on molecular vibrational-rotationaltransitions. The combustion produces adequate amounts of both carbondioxide and vibrationally excited nitrogen. The spent gases are thenexhausted to the atmosphere.

In embodiments of my invention employing electron impact excitation ofcarbon dioxide, the parameters of the discharge in carbon dioxide aresimilar to those disclosed above for the discharge in nitrogen. That is,the voltages per unit length and the currents are such that the degreeof molecular dissociation produced is not great enough to interfere withthe generation and coupling out of radiation from thevibrational-rotational transitions near 10 microns. More especially, itis important to maintain a desired concentration of undissociatedmolecules of carbon dioxide.

In order to improve the power output and efficiency of the C0 lasersdescribed above, it is important to (a) increase the depopulation ratefor the lower laser level (001 or 020) or (b) increase the rate at whichCO molecules are excited to the upper laser level (001). The additivegases (in addition to N, and CO described in the patent applicationsSer. No. 474,546 and Ser. No. 495,844 help to achieve this end.

As described in the former one of the just-cited patent applications, Ihave discovered that oxygen and water vapor are additives which areeffective to increase efficiency and power output in a molecular carbondioxide laser, as illustrated in FIG. 4.

In FIG. 4, a laser operated according to the present invention includedan interaction region comprising a tube 20 in which the lasing actionwas obtained, apparatus for furnishing therein a gas mixture containingadditives as I have discovered, and apparatus for exciting the gasmixture to enable the stimulated emission of radiation therefrom.

Illustratively, the gas mixture was furnished by a continuous flowthereof from suitable sources 41-44 of carbon dioxide, nitrogen, oxygenand water vapor, respectively. In several of the experiments, portionsof the nitrogen, oxygen and water vapor were provided by the inflow ofmoist room air. For these experiments, sources 42, 43 and 44 may be moreconveniently described as a source 46 of auxiliary gases, even though asubstantial supplementary flow of nitrogen gas was provided from a tankof gaseous nitrogen in some of these experiments.

In any event, the gases were flowed through mixing regions 32 and inletapparatus 12 into the interaction region; and the spent gases wereremoved from the interaction region to the exhaust 40 by a pump 31.

The gas mixture was excited by electrical discharge apparatus disposed,for convenience, in one branch of the inlet apparatus 12. Specifically,the discharge apparatus included the oxide-coated cathodes 14 disposedin bulbous appendages 19 of apparatus 12 near opposite ends of tube 20,the anode 15 disposed in the bulbous appendage 17 of tube 20, the DCvoltage sources 13 with positive terminals connected to anode 15 throughcurrent-limiting resistors 16 and negative terminals connected tocathodes l4, and the heater current sources 22 connected across cathodes14. Because the anode 15 is disposed in the interaction region of tube20, the discharge runs directly through the interaction region, in whichthe lasing action is obtained.

To increase the so-called population inversion between the upper andlower laser levels, i.e., the energy states associal d with thestimulated radiation, it is desirable to increase the difference betweenthe so-called rotational temperature, the molecular temperature and thevibrational temperature of the carbon dioxide. At the normal pressuresof operation of carbon dioxide and other molecular lasers of this type,the rotational temperature and molecular temperature are very nearlyequal. Thus we now want to obtain a high vibrational temperature but alow rotational temperature. To this end, the tube 20 was surrounded by ajacket 21 through which was flowed a coolant from source 26 to exhaust27. Tap water at 15 C. was used in the experiments described in detailherein; but coolants capable of depressing the rotational temperature ofthe gas even further below room temperature were still moreadvantageous. The coolant determined the wall temperature of tube 20 andthus depressed the rotational temperature of the carbon dioxide. Itappeared that the vibrational temperature was not substantially affectedby the coolant.

To achieve oscillation, it is important to include the interactionregion in an optical resonator. In the specific design being discussed,the resonator was formed by a pair of mirrors 33 and 34, the latterconcave with l 1 meters radius of curvature and the former convex withmeters radius of curvature in order to enlarge the mode volume to usethe entire gas mixture within the interaction region for laser action.The mirrors were spaced at 240 centimeters. The mirrors were supportedby suitable mirror mounts 35 and 36, respectively, and were coated to beopaque with vacuum-deposited gold. Energy was coupled out from theresonator through a 12.5 millimeter aperture in the center of concavemirror 34.

If the laser be intended for use as a straight-through amplifier, theresonator becomes unnecessary; and the mirrors can be eliminated. Insome instances, it may be preferred to locate the mirrors outside theinteraction region, in which case Brewster-angle windows (for example,of crystalline zinc sulfide, barium difluoride or potassium chloride)can be used to confine the interaction region. Such a modification ofthe embodiment of FIG. 4 including windows 47 and 48 of such materialsis shown in FIG. 6. It should be apparent that the substrates of thepartially transmissive mirrors advantageously may be of such materials.

The tube 20 in which the multiple gases interacted in the embodiment ofFIG. 4 was a glass tube, although it could be some other rigid tubularstructure, such as a tube of nonconducting plastic or quartz. The glasstube 20 extended the distance between mirrors 33 and 34 and had aninside diameter of 25 millimeters. The effective interaction region wasobserved to extend between the points at which apparatus 12 enters tube20 and was about 2 meters long.

Cathodes 14 were oxide-coated cathodes having 220 nickel bases andcontaining barium carbonate and strontium carbonate in the respectiveproportions 55 percent-45 percent. The nickel bases inherently includedsuch activator impurities as Si, Ti, Aland Mg in very small amounts.

Laser oscillation was obtained at a number of wavelengths involvingP-branch rotational transitions P(l4) to P(26), primarily P(I8), P(20)and P(22). The strongest transition, corresponding to P( 20), occurredat 10.5915 microns.

For each gas mixture, the laser power output was measured by acalibrated thermopile disposed beyond the aperture of concave mirror 34.It should be understood that the output stimulated radiation has avariety of uses and that the calibrated thermopile could illustrativelybe replaced by a desired utilization apparatus 25, which could, forexample, be an electro-optic modulator responsive to an informationsignal. The invention is particularly useful for communication, inasmuchas the atmosphere has relatively low attenuation at 10.6;2. Thewavelength range near to 10.6;1. is commonly called an atmosphericwindow.

One specific example of the operation of my invention was obtained bymixing carbon dioxide having a pressure of 0.4 Torr with room air havinga partial pressure of about 1.0 Torr made up essentially of about 0.7Torr N 0.3 Torr O, and 0.04 Torr H 0 and with additional nitrogen gashaving a partial pressure of about 1.0 Torr. Tap water was flowedthrough jacket 21 at C. With each of sources 13 providing 50milliamperes of current at 4,000 volts, i.e., about 400 watts of power,a continuous-wave power output of 16.2 watts from the laser was measuredby the calibrated thermopile. Other experiments, which involved flowingalcohol through jacket 21 at temperatures and rates that providedrotational temperatures down to 40 C. in the gas mixture within tube 20,provided improved efficiency and power in direct relation to the amountby which the rotational temperature of the gas was depressed below thevibrational temperature of the gas mixture.

When the input air was dried with calcium chloride to remove the watervapor, the coolant being tap water at 15 C., a power output of 12 wattswas obtained.

For purposes of comparison, measurements were made for a nearly optimummixture of CO, and N with no other additives. With the partial pressureof nitrogen approximately equal to 0.4 Torr and the partial pressure ofcarbon dioxide approximately equal to 0.4 Torr, the coolant being tapwater at 15 C., a power output of 4.5 watts was obtained.

It should be particularly noted that the nitrogen partial pressure wasmuch lower in the mixture without additives than in the previouslydescribed mixtures with additives and, if raised beyond about '1 .0 Torrwithout additives, tended to reduce the power output. For the systemwithout additives, the power output and efficiency were observed to befairly constant for nitrogen pressures between 0.4 and 1.0 Torr.However, the additives oxygen and water vapor permitted the nitrogenpartial pressure to be increased further with a corresponding increasein power output.

It appears that the oxygen partial pressure within the interactionregion should be greater than 0.1 Torr for an appreciable effect inincreasing the efficiency of the laser, but less than 0.5 Torr. For anappreciable additional effect from the water vapor, its partial pressurewithin the interaction region should be greater than 0.01 Torr; but lessthan 0.1 Torr. The carbon dioxide pressure should be greater than 0.1Torr and less than 1.0 Torr; and the nitrogen pressure should be greaterthan 0.1 Torr and less than 10.0 Torr.

An alternative embodiment of my invention would employ an N -N O gasmixture with O and H O additives, the excitation of nitrous oxide forlasing action being carried out in substantially the same way as theexcitation of carbon dioxide for lasing action.

Further experiments relating to my invention are described in myarticles in the Applied Physics Letters, July, 1965, and in theProceeding of the Physics of Quantum Electronics Conference, San Juan,Puerto Rico, June, 1965.

I suggest, without wishing to limit my invention hereby, that a varietyof theoretical considerations may be applicable, in varying degrees, tothese observed results.

For the first such consideration, reference is made to FIG. 2, whichshows the vibrational level v=l and two other higher excited levels fornitrogen with respect to the ground state of nitrogen. There have alsobeen shown the vibrational levels of carbon dioxide of interest, withrespect to its ground state (000). It can be seen that the N (v=l) levelat 2330.7 cm. is in very close coincidence with the (00l) vibrationallevel of CO, at 2349.16 cm.. Thus, a collision of the second kind canhave a large cross section because of the near-perfect coincidence. Thelower laser level (100) on the other hand is removed from N,(v=l) bymore than 900 cm. and hence the cross section for exciting the CO groundstate molecules to the lower laser level will be very much smaller.Moreover, the excitation of CO, (000) molecules to level (100) involvesa reaction which includes transitions which are optically forbidden.Thus, when the excited nitrogen molecules transfer energy to the carbondioxide molecules, a selective excitation of CO ground state moleculesto the (00l) levels takes place. Moreover, since the lifetimes of the(00l) levels are longer than the lifetimes of the (100) levels, therequired conditions for laser action on the (001)( 100) transitions aresatisfied. The shorter lifetime of the lower level is associated with hefact that the population of the lower level relaxes to the ground statethrough collisions in cascade fashion as depicted schematically.

An important point in the process just described is the efficientexcitation of the nitrogen (v=l) vibrational level. It is known that itis relatively difficult to excite this level directly by electronimpact. Accordingly, it is consistent with the observed results that theelectron discharge in tube 20 excites, by impact, the higher excitedlevels in the nitrogen, as indicated. The population density of theseshort-lived excited levels then decays through spontaneous radiativetransitions to populate the v=l level, as indicated.

Vibrational excitation of N; by electron impact also occurs through acompound state of N 2 existing at about 2.3 ev. (frequency, about 18,400cm.) while the vibrational excitation of CO by electron impact occursthrough a compound state of CO, existing at about 1 ev. (frequency,about 8,000 cm). Thus, it is important to have as many electrons aspossible which have their kinetic energies ranging from l- 2.3 ev.

Lower efficiencies would tend to occur in such a laser without additivesfor the following reason. In a mixture of CO and N without additives theelectron density in the discharge as a function of electron energy is asindicated by curve 51. The peak of this curve lies between the v=l leveland the higher excited levels, but substantially lower than about 2.3ev.; and the energy expended by the applied electric field inaccelerating these median-energy electrons is wasted, since they cannotexcite any of the nitrogen levels and may pass out of the interactionregion without being of use in the excitation of nitrogen or the upperlaser level of the carbon dioxide.

A possible distribution of electron energies in the discharge withoxygen is shown by curve 52. Since the vertical axis of these curvesrepresents energy and the horizontal axis represents relative numbers ofelectrons, it is seen that a substantial number of electrons would beshifted to higher energies.

Thus, it is possible that, with the addition of oxygen, the medianelectron energy, or electron temperature, as it is sometimes called,will move to higher energies until the excitation of the compound stateof nitrogen establishes an equilibrium, as indicated by the dashed curve52 of FIG. 2. Moreover, this tendency of oxygen, or an electronegativeelement, to raise the electron temperature would account for the factthat the total nitrogen pressure can be usefully increased, maintainingthe ratio of electron temperature to gas pressure at a selected level,inasmuch as a sufficient number of highenergy electrons are nowavailable to excite the compound state of substantially all of thenitrogen in spite of the increased number of electron-moleculecollisions that tend to reduce electron energy. The horizontaldisplacement or bulge in each curve indicates relative numbers ofelectrons at the different energies. The greater numbers of electrons atenergies exceeding or equal to the energy of the compound state ofnitrogen are much more effective in populating that nitrogen energylevel than are the lower energy electrons. The excited compound state ofN, then decays efficiently to the nitrogen energy level which iseffective in pumping carbon dioxide molecules to enable the 10.6 micronstimulated emission of radiation.

As illustrated in FIG. 7, a laser operated with helium added to the gasmixture included the interaction region comprising a tube 20 having aninternal diameter of at least 2 inches in the portion in which thelasing action was obtained, apparatus for furnishing therein anappropriate gas mixture and apparatus for exciting the gas mixture witha direct-current discharge with a voltage substantially directly relatedto the helium partial pressure, as shown in the table of examples below.

illustratively, the gas mixture was furnished by a continuous flowthereof from suitable sources 41, 42 and 43 of carbon dioxide, nitrogenand helium, respectively. The gases were flowed through mixing regions32 and inlet apparatus 12 into the interaction region; and the spentgases were removed from the interaction region to the exhaust 40 by apump 31.

The gas mixture was excited by electrical discharge apparatus disposed,for convenience, in one branch of the inlet apparatus 12. Specifically,the discharge apparatus included the oxide-coated cathodes 14 disposedin bulbous appendages 19 of apparatus 12 near ends of tube 20, the anode15 disposed in the bulbous appendage 17 of tube 20, the DC voltagesources 13 with positive terminals connected to anode 15 throughcurrent-limiting resistors 16 and negative terminals connected tocathodes 14, and the heater current sources 22 connected across cathodes14. Because the anode 15 is disposed in the interaction region of tube20, the discharge runs directly through the interaction region, in whichthe lasing action is obtained.

To increase the so-called population inversion between the upper andlower laser levels, i.e., the energy states associated with thestimulated radiation, it is desirable to increase the difference betweenthe so-called rotational temperature and the vibrational temperature ofthe carbon dioxide. To this end, the tube 20 was surrounded by thejacket 21 through which was flowed a coolant from source 26 to exhaust27. Water at 15 C. was used in the experiments described in detailherein but any other coolant capable of depressing the rotationaltemperature of the gas below room temperature could be used, forexample, methanol at 78 C., as disclosed in my above-cited copendingapplication. The coolant determined the wall temperature of tube 20 andthus depressed the rotational temperature of the carbon dioxide. Itappeared that the vibrational temperature was not substantially affectedby the coolant.

To achieve oscillation, it is important to include the interactionregion in an optical resonator. In the specific design being discussed,the resonator was formed by a pair of mirrors 33 and 34, the latterconcave with 50 meters radius of curvature and the former convex with48.5 meters radius of curvature in order to enlarge the mode volume touse the entire gas mixture within the interaction region for laseraction. For the best results achieved thus far, energy was coupled outfrom the resonator through a 15 millimeter diameter aperture in thecenter of convex mirror 33. The mirrors were spaced 300 centimetersapart and were supported by suitable mirror mounts 36 and 35,respectively; and they were coated to be opaque with vacuum-depositedgold. The aperture diameter may vary from any practical lower limit,such as 1.0 millimeter up to 25 millimeters while still obtainingsubstantially improved results according to the present invention, ascompared to previous vibrationally excited gas mixtures. In anotherembodiment the optical resonator includes a pair of concave mirrorshaving 50.8 centimeters radius of curvature, and the energy was coupledfrom the optical resonator through a 1.5 centimeter diameter aperture inthe center of one of the mirrors. The spacing of the mirrors remainssubstantially the same. Because of possible lens effect in the flowinggases because of cooled laser tube walls, apparently it is not possibleto calculate the mode volume from the confocal resonator theory.

If the laser be intended for use as a straight-through amplifier, theresonator becomes unnecessary; and the mirrors can be eliminated. Insome instances it may be preferred to locate the mirrors outside theinteraction region, in which case Brewsterangle windows (for example, ofcrystalline zinc oxide, barium fluoride or potassium chloride) can beused to confine the interaction region. If the laser be used in the ringlaser form, three or more mirrors will be used to form the ring.

Another use for a large diameter laser is as a multipass amplifier. Inthis case, the mirrors may be disposed to define nonoverlapping pathsfor the beam in the tube.

The tube 20 in which the multiple gases interacted in the embodiment ofFIG. 6 was a glass tube, although it could be some other rigid tubularstructure, such as a tube of nonconducting plastic or quartz. The glasstube 20 extended the distance between mirrors 33 and 34 and had aninside diame ter of 3 inches. The effective interaction region wasobserved to extend between the points at which apparatus 12 enurs tube20 and was about 2.5 meters long.

Cathodes 14 were oxide-coated cathodes having platinum bases andcontaining barium carbonate and strontium carbonate in the respectiveproportions 55 percent and 45 percent by weight.

Laser oscillation was obtained at a number of wavelengths involvingP-branch rotational transitions P(14) to P(26), primarily P(18), P(20),P(22). The strongest transition corresponding to P(20) occurred at10.5915 microns.

The laser power output was measured by a calibrated thermopile disposedbeyond the aperture of convex mirror 33.

It should be understood that the output stimulated radiation has avariety of uses and that the calibrated thermopile could illustrativelybe replaced by a desired utilization apparatus 25, which could, forexample, be an electro-optic modulator responsive to an informationsignal. The invention is particularly useful for communication, inasmuchas the atmosphere has relatively low attenuation at 10.6 microns. Thewavelength range near 10.6/.4. is commonly called an atmospheric window.

The best specific example of the operation of my invention obtained inthe early experiments involved a mixture in which the carbon dioxide hada pressure of 0.33 Torr. the nitrogen had a pressure of l .0 Torr andthe helium had a pressure of 6.0 Torr Water was flowed through acket 21at l Centigrade. Each of sources 13 provided 120 milliamperes of currentat 4,400 volts, i.e., about 1,058 watts of power; and a continuous-wavepower output of 133 watts from the laser was measured by the calibratedtherrnopile. The overall efficiency was 12.5 percent.

Various specific examples of the operation of the invention were asfollows, example 8 being thatjust given:

the design parameters are interrelated so that in most instances theoptimum mixture for a devised tube design is best determinedexperimentally consistent with the principles outlined. Numerous andother arrangements can readily be devised in accordance with theseprinciples by those skilled in the art without departing from the spiritand scope of the invention. In particular, other arrangements can beused for exciting the gas discharge, such as a radio frequencydischarge.

In all of the foregoing embodiments, the gases designated to be presentin each of the lasers is present in an amount at least two orders ofmagnitude greater than trace amounts. In lasers, such as the well-knownhelium-neon laser, trace amounts are Examples CO2, torr 0. 5 0. 2 0. 350. 4 0. 33 0. 33 0.33 0. 33 N2, torr. 2. 5 1.0 1. 0 1. 0 1. 00 1.01.0 1. 0 He,t0rr 4.67 5.0 3.5 4.00 5.0 5.0 6.0 Total mixture flow rate,liters/sec 3 3 3 3 3 3 3 3 Aperture dla. 10 10 10 10 15 15' 15 Coolanttemp. 0-... 15 15 15 15 15 15 15 15 Coolant flow rate, gpm 10 10 10 1010 10 10 10 Cathode total heater power, watts 1 200 1 200 l 200 l 200 1200 1 200 1 200 1 200 Pum ing voltage:

1 (volts) 2. 700 3, 800 3, 800 3, 800 4, 000 4, 800 4, 700 4, 400 V;(volts).. 2. 700 3. 800 3, 700 4. 000 3, 600 3, 900 4, 000 4, 400Pumping current:

I, (11111.). 87 160 160 180 150 136 130 120 I: (11121.). 87 140 170 170185 196 150 120 P s no 474 1,140 1, 231 1,124 1, 265 1, 415 1,210 1,058Output power: W1 and W2, watts. 15. 0 55.0 53. 6 s2, 5 1 11 10g 133 3. 24. 8 4. 3 7. 1 8. 2 8. 2 9. 0 12. 5

Eflicloney percent 1 100 watts each.

Higher gas pressures can be used if the discharge voltage is raised indirect relation to the pressure.

I believe that an important aspect of my present invention resides in mydiscovery that the tube readily may have an internal diameter greaterthan about 2 inches and in my discovery that the pressure ratios at agiven tube diameter are important for optimum beneficial effect of thehelium. The significant factor in the relationship of the tube diameterand gas pressure is preventing deactivation of vibrationally excitedspecies at the tube wall. Thus, if smaller tube diameters are used, tomaintain the efficiency high, there should be increased pressures of thegases, particularly CO, and helium.

One modification of the present invention involves the use of nitrousoxide as the active gas instead of carbon dioxide. Because of thecloseness of certain of its energy levels, useful for laser action, tothe corresponding energy levels of carbon dioxide, all of the foregoingconsiderations stated as applicable to carbon dioxide are alsoapplicable to nitrous oxide. Previous laser experiments with both carbondioxide and nitrous oxide indicate that these gases are sufficientlysimilar that the useful ranges of working pressures of nitrous oxide incombination with nitrogen and helium would be approximately the same asthe corresponding range of pressures of carbon dioxide. Similarly, theuseful ranges of pressure for the helium and the nitrogen should beapproximately as described above.

It is now understood that one beneficial role of water vapor and heliumin lasers according to my invention resides in their ability to relaxthe excitation of molecules having the vibrational energy of the lowerlaser leveLHelium is also important for raising the average electronenergy in the discharge, in a manner similar to that shown for oxygen inFIG. 4.

In all cases, it is understood that the above-described arrangements areillustrative of a small number of the many possible embodiments that canrepresent applications of the principles of the invention. it can beappreciated that many of considered to be amounts less than about 10parts per million by partial pressure.

lclaim: 1. An infrared laser comprising: an active medium includingcarbon dioxide, means for selectively populating a firstvibrational-rotational level of said carbon dioxide, whichvibrationalrotational level is optically connected to a lowervibrational-rotational level via radiation at about 10 microns, toestablish a population inversion therebetween, and means for resonatingsaid radiation to produce coherent radiation at about 10 microns,including means for abstracting a portion of said co erent radiation.

2. A laser according to claim 1 adapted for oscillation on transitionshaving wavelengths near 10.6 microns.

3. An infrared laser comprising: an active medium including carbondioxide, means for selectively populating a first vibrational-rotationallevel of said carbon dioxide, which vibrationalrotational level isoptically connected to a lower vibrational-rotational level viaradiation at about 10 microns, to establish a population inversiontherebetween, and means for resonating said radiation to producecoherent radiation at about 10 microns, including means for abstractinga portion of said coherent radiation, the means for selectivelypopulating the first vibrationalrotational level of carbon dioxidecomprising means for forming spaced first and second regions, means forflowing nitrogen gas through said first region, means for heating saidnitrogen gas in said first region to a temperature producingsubstantially no ionization of said nitrogen to populate an excitedlevel thereof, means for flowing said carbon dioxide gas and said heatednitrogen gas through said second region to mix and thereby to populatesaid first vibrational-rotational level of said carbon dioxide gas, theresonating means being associated with said second region.

2. A laser according to claim 1 adapted for oscillation on transitions having wavelengths near 10.6 microns.
 3. An infrared laser comprising: an active medium including carbon dioxide, means for selectively populating a first vibrational-rotational level of said carbon dioxide, which vibrational-rotational level is optically connected to a lower vibrational-rotational level via radiation at about 10 microns, to establish a population inversion therebetween, and means for resonating said radiation to produce coherent radiation at about 10 microns, including means for abstracting a portion of said coherent radiation, the means for selectively populating the first vibrational-rotational level of carbon dioxide comprising means for forming spaced first and second regions, means for flowing nitrogen gas through said first region, means for heating said nitrogen gas in said first region to a temperature producing substantially no ionization of said nitrogen to populate an excited level thereof, means for flowing said carbon dioxide gas and said heated nitrogen gas through said second region to mix and thereby to populate said first vibrational-rotational level of said carbon dioxide gas, the resonating means being associated with said second region. 